StartDate              closeDate            Hours   
'2012-12-18 08:00:00','2012-12-18 10:00:00'  2
'2012-12-18 07:00:00','2012-12-18 10:00:00'  2
'2012-12-18 07:00:00','2012-12-18 20:00:00'  10
'2012-12-18 08:00:00','2012-12-19 10:00:00'  12
'2012-12-18 07:00:00','2012-12-20 10:00:00'  22

DECLARE @timeDiff TABLE(StartDate  datetime, closeDate DATETIME)
insert into @timeDiff
SELECT '2012-12-18 08:00:00','2012-12-18 10:00:00'   union all
SELECT '2012-12-18 07:00:00','2012-12-18 10:00:00'   union all
SELECT '2012-12-18 07:00:00','2012-12-18 20:00:00'  union all
SELECT '2012-12-18 08:00:00','2012-12-19 10:00:00' union all
SELECT '2012-12-18 07:00:00','2012-12-20 10:00:00' 

;with cte AS
(
	SELECT CASE WHEN DATEPART(HH, startDate) < 8 THEN 8 ELSE DATEPART(HH, startDate) END startTime,
	CASE WHEN DATEPART(HH, closeDate) >= 18 THEN 18 ELSE DATEPART(HH, closeDate) END EndTime,
	DATEDIFF(dd, startDATE, CloseDate) noOfDays,
	StartDate,
	closeDate
	FROM @timeDiff
)SELECT (endtime *(noOfDays+1) - startTime) , StartDate, closeDate FROM cte